1. Field of the Invention
The present invention relates to a synchronization control circuit mounted on a mobile station in a TDMA (Time Division Multiple Access) cellular communication system.
2. Description of the Prior Art
There is an increasing demand for mobile communication services using automobile telephones and portable telephones. To meet this demand, it is necessary to increase the number of mobile stations which can be accommodated in a system while improving communication quality. On the other hand, a cellular mobile communication system entirely digitized by a TDM scheme is now under development for introducing various kinds of new services. In a TDM cellular communication system, each mobile station monitors, in a time slot other than the one assigned to a base station of a cell where it is located, the reception levels of carriers (referred to as frequency channels hereinafter) received from the other base stations and switches, if necessary, the frequency channel thereof to one assigned to the base station of an adjacent cell. Since the change in frequency channel causes the time base to shift, the mobile station has to be provided with means for defining a plurality of time bases matching the frequency channels of a plurality of adjacent base stations. Such means has customarily been implemented as a synchronization control circuit having a time base counter constituted by a counter for counting clock pulses having a predetermined period, and a counter controller for controlling the initialization of the time base counter and the read-out of a count.
It is a common practice with the above-described synchronization control circuit to arrange a one-to-one relationship between counter controllers and time base counters. Such a configuration makes it difficult to reduce the size, weight and power consumption of the mobile station. This is especially true with a portable mobile station which has to be miniature and consume a minimum amount of power.